1. Field of the Invention
The present invention relates to an ultrasonic sensor for detecting an object. For example, a sending element sends ultrasonic waves to an object, the object reflects the ultrasonic waves, and a receiving element in an ultrasonic sensor receives the reflected ultrasonic waves.
2. Description of Related Art
An ultrasonic sensor is mounted to an automobile, and a two-way transmittable element in the sensor sends ultrasonic waves to an object. The object reflects the waves, and the two-way transmittable element receives the reflected waves. Thus, a position or a distance for the object around the automobile can be measured. Further, a two-dimensional shape or a three-dimensional shape of the object can be measured. That is, the ultrasonic sensor is used for a safe driving by monitoring surroundings of the automobile.
For example, an automatic parking supporting system using an ultrasonic sensor is practically used. In the system, a back-sonar is used for detecting a human or an obstacle existing in the back of an automobile. The ultrasonic sensor is mounted to a rear part of the automobile, and receives ultrasonic waves reflected by the human or the obstacle. Thus, a back side collision with the human or the obstacle can be reduced.
Further, by using a micro electro mechanical system (MEMS) technology, an oscillating portion made of a piezoelectric membrane is formed on a membrane portion in a substrate, as an element for an ultrasonic sensor. Here, when the element is mounted to an automobile in an exposed state, a distance to an object to be detected may not accurately be measured, because water drops or dust may easily adhere onto a surface of the element. Moreover, the element may be damaged and destroyed by a load of an external force, e.g., a collision with a small stone.
JP-A-2002-58097 discloses an ultrasonic sensor having a protective construction for reducing the adhesion and the destruction. In the sensor, a piezoelectric element for detecting ultrasonic waves is disposed in an aluminum case so as not to be exposed to an outside. The piezoelectric element is directly mounted to an oscillating board, and the sensor receives the waves by oscillations of the oscillating board.
However, a mechanical strength of the piezoelectric membrane as the element produced by the MEMS technology is low due to its structure. When the element is directly mounted to the oscillating board, the element may easily be destroyed. By contrast, if a space is provided between the piezoelectric membrane and the oscillating board in order to reduce the destruction, the waves may not effectively be transmitted to the element.
Furthermore, when a resonance oscillation of a receiving portion is used for transmission of ultrasonic waves, a thickness of an oscillating board is determined by a size and a material of the receiving portion, and a frequency of the ultrasonic waves. Therefore, when an ultrasonic sensor includes a small element, an oscillating board in the small element has to be thin. For example, if a wave frequency is 50 kHz, and if an aluminum square board, 3 mm on a side, is used as the receiving portion, a thickness of the board has to be equal to or less than 0.1 mm. In this case, strength of the board cannot be secured. If the thickness is increased in order to secure the strength, a signal intensity may be too low to be detected, because an amplitude of an oscillation becomes small.